When Facebook announced hashtags, you knew it was only a matter of time until the social network announced trending topics. And following testing on the web and mobile in August, Facebook today launched a redesigned version of the “Trending” section on the web sidebar in the US, UK, Canada, India, and Australia. It showcases personalized lists, based on what you’ve liked and what’s trending across the social network, of the most mentioned words and phrases of the moment, with explanations of why each os trending. The idea is to spur public sharing, drive return visits, and make Facebook a place to go for trending current events – in the way that Twitter is at present. Facebook’s “Trending” section is different from Twitter’s in that there is a reason offered to users for why the trend is happening – as opposed to just a hashtag – to provide a context for the user so that they don’t leave the site to do a web search. And by keeping users on the site longer via these current events topics, it hopes to drive revenue derived from users clicking and sharing ads – an important category of revenue in its post-IPO world.